@Jules ******
>My partner and I are not married yet, but do you know if we can legally do so in Thailand
You should be able to legally get married. You will need to go to your embassy and ask them for a freedom of marriage certificate or a letter to give to the Thai government saying you are able to be married. You'll get that letter translated at the Ministry of Foreign Affairs. You'll go to the local Amphur office (the "city hall", all large cities have at least one), and ask them for the requirements to get married. You'll submit the paperwork and get married.
Once you're married, you need to talk to the local immigration office about your visa options. You MAY need to get a Non-O visa based on marriage to a Thai national. This would be 2K THB, and then you'd have to pay another 1,900 THB for a one year extension of stay. There is a possibility that you can get a one year extension of stay based on marriage straight from your current extension. That would be 1,900 THB.
There is *NO* financial requirements for a one year extension of stay based on marriage to a male Thai national, or a Non-O visa based on marriage to a male Thai national.